Biochemistry Module 6 Practice Exam

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In hepatic metabolism, how do insulin and glucagon regulate PFK-1 activity via PFK-2 and fructose-2,6-bisphosphate?

Insulin activates dephosphorylation of PFK-2 increasing F2,6BP and PFK-1 activity; glucagon activates phosphorylation lowering F2,6BP and PFK-1 activity

Insulin causes phosphorylation of PFK-2; glucagon causes dephosphorylation

When insulin and glucagon regulate hepatic glycolysis, key control is through PFK-2 and the level of fructose-2,6-bisphosphate (F2,6BP), which is the potent allosteric activator of PFK-1. PFK-2 is a bifunctional enzyme: its kinase activity makes F2,6BP, and its phosphatase activity degrades it. The balance between these activities is set by the phosphorylation state of PFK-2. In the liver, dephosphorylated PFK-2 acts as a kinase, raising F2,6BP and thereby increasing PFK-1 activity to promote glycolysis. Phosphorylated PFK-2 acts as a phosphatase, lowering F2,6BP and reducing PFK-1 activity, which slows glycolysis and favors gluconeogenesis.

Thus insulin, by promoting dephosphorylation of PFK-2, increases F2,6BP and enhances PFK-1 activity. Glucagon, by promoting phosphorylation of PFK-2, decreases F2,6BP and lowers PFK-1 activity. This explains why insulin drives glycolysis while glucagon shifts metabolism toward gluconeogenesis.

Both hormones increase F2,6BP

Glucagon has no effect on PFK-2